This invention relates to an apparatus and method for determining the position of a movable element in a data space, and to improvements in techniques for determining the travel time of acoustic energy over an unknown travel distance between an acoustic transmitter and an acoustic receiver.
Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element (such as a stylus or cursor) and one or more transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element in one, two, or three dimensions, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. The transit time of sound traveling from a source on the movable element to each of a plurality of microphones is used, in conjunction with the velocity of sound in air and known geometrical relationships, to compute the position of the movable element.
The accurate determination of the transit time of the acoustic energy between the transmitter and receiver locations is critical to an accurate determination of the position of the movable element. Typically, a timer is provided for each receiver. All of the timers are started when the acoustic energy is transmitted from the transmitter. As the sound is received at each receiver, the timer associated with that receiver is stopped. The transit times to each receiver can then be computed from the time that elapsed on each timer. Typically, each timer is a digital counter which counts pulses from a digital clock generator, and the arrival of acoustic wave energy at each microphone is determined by continuously comparing the microphone output (e.g. an amplified and filtered version thereof) to a predetermined threshold level. When the threshold level is exceeded, the associated counter is turned off.
In the described type of system, a good source of acoustic wave energy pulses is a spark gap which is energized by triggering a circuit that delivers voltage pulses to a pair of closely spaced electrodes which comprise the spark gap. The trigger pulse for this circuitry is also conventionally utilized to initiate the timer or timers that are employed to measure the transit time of the acoustic wave energy over an unknown distance to be determined. [As noted above, the timers are subsequently terminated when the acoustic wave energy is received at one or more respective receivers. The measured elapsed time can be used for determination of distance or, for pilot purposes, by determination of the velocity of sound in air when the transmitter to receiver distance is known.] The spark does not occur immediately upon application of the trigger signal to the spark generation circuitry, so the timer(s) may be initiated somewhat prematurely, resulting in an incorrect elapsed time measurement. This would not necessarily be problematic if one could determine the precise time relationship between application of the trigger pulse and occurrence of the spark, since suitable correction could then be applied to the measured elapsed time. Applicant has found, however, that such solution is generally not adequate, since the time between the trigger and the actual spark can vary considerably. There is a build-up time of the voltage across the electrodes before a spark is produced (generally, at the output of the a transformer which is part of the spark generation circuitry). The build-up may not be the same for each spark to be generated and, also, the voltage at which a spark is produced can vary over the life of the electrode pair, and can also vary for different electrode pairs. This means that the timing error will tend to vary and cannot be readily accounted for by adding a predetermined timing correction.
It is among the objects of the present invention to provide solution to the problem as set forth.